The young at their birth have only four feet, and their body is ronuded and tailless. In this state they are the genus Amymonc of Mtiller. Some time afterwards (in about fifteen days in the months of February or March) they acquire another pair of feet ; they are then the genus .Nauplius of the same author. After their first moult they assume the form and all the parts which characterise the adult state, but with smaller proportions: their antennae and feet, for example, are comparatively short. At the end of two more metals they are fit for the reproduction of the species. The greater part of these &do mostraca swim upon their backe, darting about with vivacity, and possessing the power of moving either backwards or forwards. Their food generally consists of animal matter in preference to vegetable ; but in the absence of the former they feed upon substances of the latter description, and it is said that the fluid in which they live never enters their stomachs. The alimentary canal extends from one extremity of the body to the other. The heart (taking Cyclops Castor as the subject) is of a shape approaching to oval, and situated imme diately under the second and third segment of the body. Each of the extremities of this organ gives off a vessel, the one going to the head, the other to the tail. Immediately below is another analogous organ, giving off also at each end a vessel supposed to represent the branehiocardiac canals Observable in the circulation of the Decapodoue Crustacca.
The genus Cyclops is an inhabitant of the fresh waters ; and we select the Common Cyclops, Cyclops vulqaris, Leach ; Monoculus quadricornis, Linn.; Cyclops quadricornis,111iiller ; Monocle h Queue Fourehue, Geoffrey, as an example of the species.
The body of the Common Cyclops has a somewhat swollen appear ance, and is formed of four rings, and prolonged to about one-third of its entire length. The tail consists of seven rings. The posterior antenna= (antennules of Jurine) are tolerably large and composed of four joints, the anterior antennae are thrice the length of the posterior. There are several varieties.
Var. a. Reddish ; eggs brown, forming two oblique massea near the aides of the tail. Total length eight-twelfths of a line. This is the Monoculus quadricornis rubens of Jurine.
Var. b. Whitish or gray, somewhat tinged with brown, rather larger than the preceding. Egg-masses greenish, forming nearly a right angle with the tail. Total length the same as the preceding. This is the M. q. albiclus of Jurine.
Var. c. Greenish. Direction of the two egg-masses intermediate between that of the egg-masses of the two former. Length nine twelfths of a line. H. q. viridis of Jurine.
Var. d. Smoky red. General form nearly oval. Eggs brown, com posing two masses, which cover a great portion of the tail.. Length six-twelfths of a line. H. q. fuscus of Jurine.
Var. e. Of a deeper green than var. c. Eggs obscure green, passing a little into rose-colour when hatching is near, forming two masses attached to the tail, and appearing to be incorporate with it. Length the same as the preceding. M. q. prasinus of Jurine.
Ostracoda, Latreille ; Ostropoda, Straus.
The shell of the Ostracoda is formed of two pieces or valves repre senting those of a couchiferous mollusk or bivalve shell, but horny, not testaceous. As in the bivalves, the two pieces are united by a binge, and when the animal is inactive they close upon and shut in the and the parts. The feet are ambulatory, six in number, and none are terminated by a digitated swimming organ, nor accompanied by a branchial lamina. The antennae are simple, filiform, or aetaceous. There is but one eye, which ia composite and sessile. The mandibles and jaws are furnished with a branchial lamina, and the eggs are situated on the back.
In this division Latreille includes the genera Cythere, visa, Lamarck) and Cypris. Cythere has one eye; three pairs of feet;
abdomen short ; the inferior or pediform antennae furnished with one tolerably long curved and jointed filament. The species are inhabit ants of the sea. They have not the power of swimming, but are always walking among the leaves or branches of the amferven and Faci, where they delight to dwell. When shaken out of their hiding places into a tumbler or bottle, they may be seen to fall in gyrations to the bottom, without ever attempting to dart through the fluid, as would be the case with the species of Cypris. Upon reaching the bottom they open their shells and creep along the surface of the glass ; but when touched or shaken they immediately again withdraw themselves within their shell and remain motionless. Dr. Baird, whose work on the British species contains a fund of information on the habits of the minute family to which these creatures belong, says that the species " are undoubtedly numerous, and the ]abours'of any inquirer after them would assuredly be rewarded with success." He has described fifteen species as inhabitants of the British coasts. Several of the speciea have been found fossil.
Cypris has two pairs of feet, one pair always inclosed within the shell. The two antennae are terminated by a pencil of fine hairs. The case or shell is suhoval, arched, and protuberant on the back or hinge aide, and nearly straight or a little sinuous or kidney-shaped on the opposite edge. A little in advance of the hinge, and upon the mesial line, is the single large blackish round eye. The antennae, which are inserted immediately below, are shorter than the body, aetaceous, composed of from seven to eight joints, of which the last are the shortest, and terminated by a pencil of twelve or fifteen fine hairs, which serve as swimming organs. The mouth is composed of a cari nated labrum ; of two large toothed mandibles, each furnished with a feeler of three joints, to the first of which a small branchial lamina of five digitations (interior lip of Ramdohr) is attached, and of two pairs of jaws ; the two upper, which are much the largest, have on their internal border four moveable and silky appendages, and exter nally a large branchial lamina pectinated on its anterior edge ; the second are formed of two joints, with a short, nearly conical, and jointlesa feeler, also silky at the end. A sort of compressed sternum performa the office of a lower lip (external lip of Ramdohr). The feet have five joints, the third representing the thigh, and the last the tarsua ; the two anterior ones, much stronger than the rest, are inserted below the antennae, directed forwards with stiff hairs on long hooks collected into a bundle at the extremity of the two last joints : the four following feet are without these appendages. The second pair, situated on the middle of the under side of the body, are directed backwards, curved, and terminated by a long strong hook bent for wards ; the two last., never showing themselves beyond the shell, are applied to the sides of the body for the purpose of sustaining the ovaries, and are terminated by two very small hooks. There is no distinct joint obaervable in the body, which terminates posteriorly in a kind of tail, which is soft and bent upon itself underwarda, with two conic or setaceous filaments fringed with three silky hairs or hooks at the end, and directing itself backwards so as to project beyond the shell. The ovaries form two large vessels, simple and conical, situated upon the posterior sides of the body under the shell, and opening, one at the side of the other, at 'the anterior part of the abdomen, where tho canal formed by the tail establishes a communication between them. The eggs are spherical.